We report on the localisation of an impact event on a carbon fibre composite plate using three surface-mounted fibre Bragg grating (FBG) sensors to monitor ultrasonic Lamb waves. We combine a novel wavelength-intensity conversion technique that allows fast-sampling of the FBGs, and an algorithm for localising an impact on an orthotropic plate. We are capable of locating the impact to a precision of a few millimetres. To our knowledge, this is the first time this combination has been developed, which could find applications for combined health monitoring and impact localisation.